The present invention relates in general to filter circuits and, more particularly, to a low-pass filter for a monolithic integrated circuit.
Monolithic integrated circuits generally require a stable power supply voltage for proper operation. An effective method of stabilizing the power supply voltage is to low pass filter an unregulated power supply into the non-inverting input of an operational amplifier which receives its operating potential from the unregulated power supply. The output of the operational amplifier provides the regulated power supply operating within 0.5 volts of the unregulated power supply voltage. The DC response of the regulated power supply voltage is correlated to variations of the unregulated power supply voltage because the operational amplifier receives a filtered fraction of the unregulated power supply voltage.
The low-pass filter must have a low cut-off frequency, say 30-40 Hz, in order to effectively suppress AC noise from the unregulated power supply voltage. Moreover, to minimize the cost of manufacture, the low-pass filter should be located on the same chip as the circuitry utilizing the stabilized power supply voltage. The resistance and capacitance needed to form the corresponding RC time constant may for example have values of 200 megaohms with a 20 picofarad capacitor, or alternately 1 megaohm and a 4 nanofarad capacitor. The maximum practical limits for on-chip resistors and capacitors is typically 1 megaohm and 20 picofarads, respectively. Thus, the values of resistance and capacitance necessary to make the proper monolithic low-pass filter are impractical with conventional resistors and capacitors used in integrated circuits.
Hence, a need exists for a low-pass filter with a low cut-off frequency for use on a monolithic integrated circuit.